(1) Field of the Invention
The present invention generally relates to a method of depositing an insulating layer using a plasma-assisted CVD (Chemical Vapor Deposition) process, and more particularly, to a method of depositing an insulating layer on an underlying layer using a low temperature plasma-assisted CVD process employing a pulse-modulated plasma.
(2) Description of the Prior Art
Conventionally, a plasma surface processing apparatus having parallel plate electrodes is widely used as a discharge reaction apparatus. The parallel plate electrodes are placed in a vacuum chamber having a gas intake system and a gas exhaust system. A direct current signal, an alternating current signal or a high-frequency signal generated by a signal source is applied across the parallel plate electrodes so that a discharged plasma (i.e., a plasma discharge) is generated. A wafer is placed on a plate which is heated by a heater or cooled by a gas or coolant and the surface of the wafer is processed by the above-mentioned discharged plasma.
There are many variations and modifications of the above-mentioned apparatus. For example, one type of plasma surface processing apparatus uses coaxial cylinder type electrodes instead of the parallel plate electrodes. Another apparatus uses a large number of electrodes. However, these conventional apparatuses have a common disadvantage in that the density of electrons contained in the discharged plasma is low and thus it takes a long time to process the surface of the wafer. Further, a layer formed by the discharged plasma has a very high thermal stress and a very large internal stress. In addition, the stress is uniformly exerted. Moreover, since the wafer is directly in contact with the discharged plasma, it is damaged due to the projection of electrified particles.
In order to eliminate the damage due to the direct projection of electrified particles, an improved apparatus has been proposed in which a discharged plasma is generated in a place away from the wafer, and only active pieces or ions contained in the discharged plasma are transported to a processed surface of the substrate. However, the above-mentioned proposed apparatus does not have a high discharged plasma density, so that it takes a long time to process the wafer surface. In order to increase the surface processing speed, it is necessary to apply a larger amount of power to the apparatus.
FIGS. 1(A) through 1(C) show a conventional method of producing a semiconductor integrated circuit device by a plasma-assisted CVD process, and FIG. 2 shows a conventional plasma-assisted CVD apparatus. The device shown in FIGS. 1(A) to 1(C) comprises a semiconductor substrate 31 made of, for example, silicon (Si), an insulating layer 32 made of, for example, PSG (Phospho-Silicate Glass), patterned wiring layers 33 formed of Al, and an insulating layer 34 formed of, for example, silicon oxide (SiO.sub.2) and functioning as a cover (passivation) layer. The insulating layers 32 and 34 are deposited by the apparatus shown in FIG. 2 in the following way. A wafer 38 (corresponding to the substrate 31) is placed on an electrostatic chuck 37 in a vacuum reaction chamber 35. An RF (Radio Frequency) generator 40 generates a continuous wave, and a rectangular wave guide 36 transports the continuous wave to the chamber 35. The wafer 38 is heated by a heater 39.
At the production step shown in FIG. 1(A), the PSG layer 32 is deposited on the substrate 31 by a plasma-assisted CVD process in which a continuous wave is applied to the chamber 35 which is maintained at approximately 350.degree. C. At the production step shown in FIG. 1(B), an Al layer is deposited on the entire surface of the PSG layer 32 by a sputtering process, and then patterned by an RIE (Reactive Ion Etching) process step, so that the Al wiring layers 33 are formed. At the production step shown in FIG. 1(C), using the apparatus shown in FIG. 2, the SiO.sub.2 layer 34 is deposited on the entire surface by the plasma-assisted CVD process in which the continuous wave is applied to the chamber 35 and the plasma is continuously generated.
The above-mentioned plasma-assisted CVD process is superior to a normal CVD process in that the former process can be performed at a temperature (about 350.degree. C.) lower than that of the latter process (about 400.degree. C.). However, the qualities of layers formed by the plasma-assisted CVD process are not better than those of layers formed by the normal CVD process. This problem is more frequently encountered as the deposition temperature decreases.
Conventionally, the continuous plasma generation is employed in order to obtain the large amount of power necessary to deposit the insulating layers 32 and 34 at a high speed and thereby to improve the shapes of stepped parts thereof (coverage). During the deposition process, the wafer 38 is continuously biased. Thus, hydrogen groups are actively implanted in the insulating layer being deposited due to the ion bombardment effect, so that hydrogen groups, particularly O--H groups, are contained in the insulating layers 32 and 34. When the insulating layers 32 and 34 containing the O--H groups are heated, a great change in the internal stress (for example, detachment of hydrogen groups in the insulating layers 32 and 34 and a resultant bonding rearrangement) is liable to take place. Such a great change in the internal stress degrades the quality of the insulating films 32 and 34.